The compositions of individual secretory granules have been measured using electron probe x-ray microanalysis in resting, or unstimulated, pancreatic islet cells. Islets were isolated by collagenase digestion from adult rats, incubated for 2 to 3 hours in culture medium at 37 degrees C in the presence of 5.6 mmol/1 glucose, frozen, and cryosectioned. Cryosections were freeze-dried and analyzed in the electron microscope. Granules in some cells contained high levels of sulfur and zinc, attributable to insulin storage organelles in beta cells. Granules in other cells contained low levels of sulfur and no detectable zinc, attributable to glucagon-containing secretory organelles in alpha cells. Phosphorus content was high in granules of both alpha and beta cells, consistent with other observations that ATP is concentrated in secretory granules. Granule calcium concentrations typically varied from 10 to 80 mmol/kg dry weight within a beta cell. Zinc content of beta granules was approximately 30 mmol/kg dry weight and sulfur content approximately 400 mmol/kg dry weight, quantitatively consistent with other data on the zinc-insulin storage complex. Interestingly, a few granules with composition characteristic of beta granules were found in non-beta cells.